Modification of Ni-Substituted Pyrochlore Catalysts for the Steam Reforming of Methane

CATEGORY: METHANE
Modification of Ni-Substituted Pyrochlore Catalysts for the Steam Reforming of Methane
NAM24-24th North American Catalysis Society Meeting, Pittsburgh, PA, June 14-19, 2015
Daniel J. Haynes1, Dushyant Shekhawat 1, David A. Berry 1, Mark Smith 2, Devendra Pakhare 3 and James J. Spivey 4
(1) National Energy Technology Laboratory, USA, (2) URS, USA, (3) Pyrochem Catalyst Co., USA, (4) Louisiana State University, USA.
Introduction
Recent discoveries of natural gas supplies have led to an increasing interest in the reforming of methane to produce hydrogen for use in applications like chemicals and energy production. Although steam reforming of methane (SMR) is a relatively mature process, there are still significant incentives to increase the activity and stability of Ni- based catalysts that are widely used in commercial reforming processes. However, developing a Ni-based catalyst to withstand the rigors of typical reforming conditions is challenging considering the activity of Ni is plagued by numerous deactivation mechanisms;
including carbon formation, thermal sintering, and oxidation from the high steam partial pressure [1]. Previous studies have shown that the substitution of an active metal into the structure of a thermally stable pyrochlore structure can minimize the deactivation by carbon formation and thermal sintering [2]. For this study, Ni will be isomorphically substituted into the structure of the La2Zr2O7 pyrochlore, to create small, well-dispersed, and highly stable Ni sites at the surface which are active for SMR. Further, four different 1st row transition metal (TM) promoters will also be substituted into the pyrochlore structure to minimize the oxidation of Ni sites.
Materials and Methods
The substituted pyrochlore catalysts were synthesized by a variation of the Pechini method [3]. Ni loading was 6wt% for each catalyst, and the amount of TM promoter added was set to 10% of the atomic loading of Ni. SMR experiments were performed in a fixed bed continuous-flow reactor with an S/C=2.0, T= 700 °C, 2 atm, and inert gas composition of 25%. Reforming activity was evaluated as the weight hourly space velocity (WHSV) was increased from 25,000 to 200,000 scc/gcat/h. Carbon formation was quantified by a burnoff after the SMR experiment.
Results and Discussion
Hydrogen concentrations from the SMR studies for the TM promoted Ni catalysts are shown Figure 1. At the lowest WHSV, all catalysts have a product distribution near equilibrium values (~60% H2), with the exception of the Fe promoted catalyst. The activity of the Cr promoted catalyst proved to be the most active as it was able to maintain stable, equilibrium yields through 50,000 WHSV, and showed higher syngas production compared to the other catalysts over each WHSV tested. Activity decline can likely be attributed to the oxidation of Ni metal, as the post run burn off indicated that deactivation by carbon was unlikely by showing an insignificant amount of carbon (ca. 0.0095 gcarbon/gcatalyst).Given similar ionic size of each of the four TM promoters compared to Ni, it is hypothesized that the promoters behave much like Ni during the calcination treatment for the formation of the pyrochlore powder. Therefore, they would occupy a similar coordination and position in proximity to the Ni which likely results in their promotional effects. As observed by the activity test (Figure 1), it could be assumed that the Cr is more dispersed near the active Ni at the surface compared to Fe, however this will need to be confirmed by further characterization with XPS and EDX.
Significance
The addition of Cr to a Ni-substituted pyrochlore improved the activity for SMR at high space velocities by reducing the rate of deactivation, which was likely attributable to the oxidation of active Ni sites.
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